Various compositions are used in effecting a seal between a ceramic component and a metallic component of a spark plug. For example, electrically conductive, semi-conductive and resistive seal materials are used in center wire assemblies of spark plugs to provide a hermetic seal within the internal bore of the insulator and to provide an electrically conductive path between the terminal electrode and the center electrode of the spark plug. The terminal electrode and the center electrode may be directly connected to one another within the internal bore by the conductive seal material, or they may be indirectly connected by a series of electrical components that includes the conductive seal material and establishes an electrical path from the terminal electrode to the center electrode. For example, it may be desirable to seal a resistor between two electrically conductive seal layers within the internal bore of the insulator to help suppress the generation of ignition-related electromagnetic interference (EMI), also known as radio frequency interference (RFI). Additionally, the seal acts as the resistive element and helps to suppress EMI.
Electrically conductive seal materials are typically composite materials that include a combination of electrically insulating and electrically conductive materials. In general, a uniform dispersion of the insulating materials and the conductive materials is desirable. To help improve the homogeneity of powdered seal materials, these materials are sometimes formed into free-flowing granules, pellets, tablets, extrudates, or other types of aggregates, which may require addition of a liquid to the powdered materials. In such case, the granulated, pelletized, extruded, or otherwise agglomerated seal materials must be thoroughly dried before they can be introduced into an internal bore of an insulator and used to form a seal of a spark plug.